The Chevrolet Corvair engine was a flat-6 (or boxer engine) piston engine used exclusively in the 1960s Chevrolet Corvair automobile. It was a highly unusual engine for General Motors: It was air-cooled, used a flat design, with aluminum heads (incorporating integral intake manifolds) and crankcase, and individual iron cylinder barrels. The heads were modeled after the standard Chevrolet overhead valve design, with large valves operated by rocker arms, actuated by pushrods run off a nine lobe camshaft (exhaust lobes did double duty for two opposing cylinders) running directly on the crankcase bore without an inserted bearing, operating hydraulic valve lifters (which eliminated low temperature valve clatter otherwise seen with that much aluminum in the engine, due to its high degree of thermal expansion).
The flat horizontally opposed (“flat engine“) air-cooled engine design, previously used by Volkswagen and Porsche as well as Lycoming aircraft engines, offered many advantages. Unlike inline or V designs, the horizontally opposed design made the engine inherently mechanically balanced, so that counterweights on the crankshaft were not necessary, reducing the weight greatly. Eliminating a water-cooling system further reduced the weight, and the use of aluminum for the heads and crankcase capitalized on this weight reduction; so that with the use of aluminum for the transaxle case, the entire engine/transaxle assembly weighed under 500 pounds (225 kilograms). In addition, the elimination of water-cooling eliminated several points of maintenance and possible failure, reducing them all to a single point; the fan belt. As with the Volkswagen and Porsche designs, the low weight and compact but wide packaging made the engine ideal for mounting in the rear of the car, eliminating the weight and space of a conventional driveshaft.
Two years after its 1960 debut, the Corvair engine gained another unusual attribute: it was the second production engine ever to be equipped from the factory with a turbocharger, released shortly after the Oldsmobile Jetfire V8.
Aircraft hobbyists and small volume builders, perhaps seeing the Corvair engine’s similarity to Lycoming aircraft engines, very quickly began a cottage industry of modifying Corvair engines for aircraft use, which continues to this day. The Corvair engine also became a favorite for installation into modified Volkswagens and Porsches, as well as dune buggies and homemade sports and race cars.
140
The Corvair’s innovative turbocharged engine; The turbo, located at top right, takes in air through the large air cleaner at top left, passes it through the sidedraft carburetor in between, and feeds pressurized fuel/air mixture into the engine through the chrome T-tube visible spanning the engine from left to right.
The Corvair’s innovative turbocharged engine; The turbo, located at top right, takes in air through the large air cleaner at top left, passes it through the sidedraft carburetor in between, and feeds pressurized fuel/air mixture into the engine through the chrome T-tube visible spanning the engine from left to right.
The initial Corvair engine displaced 140 in³ (2.3 L) and produced 80 hp (60 kW). The high performance optional “Super TurboAir” version, introduced mid 1960 with a special camshaft and revised carburetors and valve springs produced 95 hp (70 kW).
145
In 1961, the engine received its first increases in size via a larger bore. The engine was now 145 in³ and the base engine was said to produce the same 80 hp (60 kW). The new high performance engine was rated at 98 hp (73 kW). In 1962 the high performance engine was rated at 102 hp (76 kW). The high compression 102 HP heads were added to the Monza models equipped with Powerglide when the standard engine was ordered, giving an 84 HP engine rating. 1962 engines returned to automatic chokes after a one year only manual choke on 1961 models.
The ultimate performance was found in the Spyder model, which became available with a turbocharged engine rated at 150 hp (112 kW). The turbocharger was mounted on the right side of the firewall behind the rear seat, fed by both exhaust manifolds; a single sidedraft carburetor mounted on the left side of the firewall fed directly into the turbocharger’s intake, with a chromed pipe leading from the turbocharger’s outlet to what would otherwise be the carburetor mounting pads on the intake manifolds, which were integral parts of the heads. The turbocharged heads received some valve upgrades to improve durability. Exhaust valves on turbocharged engines were made from a non-ferrous material used in jet engine turbine buckets, called ‘Nimonic 80-A’. All other Corvair engines had slight upgrades in valve and valve seat materials as well for 1962.
164
The engine was stroked out (from 2.6″ to 2.94″) displacing 164 in (2.7 L) for 1964. Power output was boosted to 95 hp (70 kW) for the base model and 110 hp (80 kW) in the high performance normally aspirated engine, while the Turbocharged engine remained rated at 150 hp for this year. This increase in stroke was the maximum the engine could tolerate, to the point that the bottoms of the cylinder barrels had to be notched to clear the big end of the connecting rods.
For the 1965 model year, all engines had the head gasket area between the cylinder and the head widened, with a new design folded “Z” section stainless steel head gasket virtually eliminating any risk of head gasket failure. A 140 hp (104 kW) version with 4 single barrel carburetors, and a progressive linkage was introduced in 1965 as option L63 ‘Special High Performance Engine’ and was standard equipment on the Corsa model. The carburetors consisted of a single barrel primary and a single barrel secondary on each head, connected by a progressive linkage; in addition, the heads featured a 9.25:1 compression ratio, and the cars received dual exhaust systems. Engines supplied with the automatic transmission after spring 1965 were modified with a camshaft from the 95 Horsepower base engine, and a special crankshaft gear that retarded its timing 4 degrees- the former to increase torque and smooth idle with the Powerglide transmission, the latter to restore some of the peak HP lost at higher engine speeds by the economy contoured camshaft with short timing.
1966 engines were basically carryover from the 1965 models, however Corvairs sold in California (except Turbocharged models) now featured the General Motors Air Injection Reactor System (AIR), and emissions control system consisting of an engine driven air pump that drew filtered air from the air cleaner, and injected a metered amount into the exhaust manifolds via tubing to promote complete oxidation and combustion of exhaust gasses to lower emissions. Specially calibrated carburetors and slight changes to the ignition timing and advance curves were part of the package. The AIR system had an unfortunate effect of sustantially raising exhaust gas, valve and head temperatures, particularly under heavy loads and this was a drawback on the Corvair where engine cooling could not be easily improved to cope with the higher temperatures. Nonetheless, performance and drivability were not noticably effected in most circumstances. In 1968, all Corvair (and other GM) engines got the AIR system for every market.
The 140 HP engine was officially discontinued for ’67, but became optional in 1967 as COPO 9551-B, not a regular production option. Chevrolet sold 279 of these engines in the 1967 model year, 232 with manual transmissions, and 47 with Powerglide transmissions. Only six were sold with the four carburetor engine and the AIR injection system required by California emissions standards. These figures include 14 Yenko Stingers and 3 Dana Chevrolet variants of the Stinger.
Both the 140 HP engines and the Turbocharged engines had many special quality features not shared with lesser Corvairs- Moly insert top rings, stellite tips and faces on the valves, a Tufftrided (cold gas hardened) crankshaft, and Delco Moraine ’400′ aluminum engine bearings- the quality of the 140HP Corvair engine for materials is directly comparable to the Rolls Royce V8 of that era, item for item. It was a fabulous bargain for the $79 premium it commanded over the basic 95HP engine. Performance of the 140HP engine was better than you might expect, with a 5200 rpm peak horsepower output, it offered road performance in a Corvair comparable to contemporary Cadillac models of the day.
The turbocharged engine now developed 180 hp (134 kW). Contemporary reviews describe a similarity in power between the turbocharged and four-carburetor engines throughout the low and mid rpm range, with the turbocharged engine being superior only when it was possible to sustain boost continously. The turbocharged engines long suit was highway acceleration, flooring the accelerator at turnpike speeds produced ferocious acceleration in the upper speed ranges as the turbocharger began to boost, reaching manifold pressures approaching 15 PSI. No wastegate was used on the Corvair turbocharged engine, boost was controlled by careful balancing of exhaust restriction, mostly via the muffler, and intake restrictions from the smallish Carter YH carburetor used. Preignition and knock under boost was controlled using a novel ‘pressure retard’ device, essentially a modified vacuum advance device, on the specially curved distributor, as boost pressures built, ignition advance was progressively reduced to preclude detonation.